Is there a table or a clear explanation of what the different size categories are? I know humans are medium, but how big/heavy is Gargantuan? Or Huge?

I'm not sure about weight, but you can find space and reach values for the size categories in Table 8-4 (linked here) of the PRD.

One way you could approach the weight issue is to look at the Enlarge/Reduce Person spells.

The enlarge person spell multiplies the target's weight by 8 and increases it by one size category. If you take an average Medium-size weight (I'll say 175 for a rough guess), then a large creature weighs about 1400 pounds, which is more than a typical horse. But since a rhino, which isalso a large creature, can easily weigh 3000 pounds, you can see there's a wide range for weight.

Similarly, using the 175-lb average, a small creature would weigh about 22 pounds, which is about right for some breeds of dog or a very overweight cat, which are considered small creatures.

A huge creature will weigh about 1400*8 = 11,200 pounds.

For a colossal creature, 175*8*8*8*8 = 716,800 pounds.

Obviously, the higher up you go, the wider the variation. Body type will make a difference, too, but this can give you a starting point, at least.

Technically if you look at some of the extensions from 3.5 you can extrapolate different sizes(Star Wars Saga has 'Colossal Plus' and then 'capital' for example) but as mentioned above colossal is anything above gargantuan in size.

As a somewhat example of that, the WotC Colossal Red Mini they had in the 3.5 days was larger than a traditional colossal sqauare. I think it was 7x7.

Craig Cochrane did a great "mega-epic" series of supplements for 3.5 that took players up to 100th level or so. You can find his stuff here. He goes into a lot of detail of much, much bigger creatures in the epic bestiary.

He scaled the game up so that you could play games at "Green Latern" level of epic play.

Craig Cochrane did a great "mega-epic" series of supplements for 3.5 that took players up to 100th level or so. You can find his stuff here. He goes into a lot of detail of much, much bigger creatures in the epic bestiary.

He scaled the game up so that you could play games at "Green Latern" level of epic play.

I'm a BIG fan of this; I use it all the time. I think the Elder Evils folks should have used something like it for Atropus, frankly.

The only thing missing for me is game rules for making really large creatures immune to stupid things like Devastating Critical, though given their insane Con values, they're really unlikely to fail the save anyways.